1. Field of the Invention
The present invention is related to an integrate forming method for the front aluminum alloy fork of a bicycle, and especially to the method wherein an aluminum alloy extrusion pipe having therein two elliptical pipe holes is used as a blank and is milled by a milling machine to form in advance a pipe casting of the front fork; then taking advantage of plasticity of the die core filled with plastic and the characteristic favorable for taking off die cores from the shaped pipe holes, the casting of the front fork can have adequate strength for the processes of forming of a pipe head, a first stage forming of branch pipes, a second stage forming and bending forging etc. to thereby make a round pipe head and two arciform branch pipes of the front aluminum alloy fork of the bicycle connect together, and thus the pipe head and the branch pipes can be integrately formed, so that strength and good appearance of the front aluminum alloy fork of the bicycle without sealing the joints with adhesive thereon can be improved.
2. Description of the Prior Art
As is known, the front aluminum alloy fork 1 of a bicycle (as shown in FIG. 1) is formed by connecting the two legs of a round pipe head 11 respectively to two tapered branch pipes 12, 13 with the diameters thereof reduced and connecting two engaging portions 15 respectively; the pipe head 11 is pivotally connected to the bottom of the handle bar stem in front of the triangular bike frame, the two engaging portions 15 on the bottoms of the branch pipes 12, 13 are used to mount a front wheel and to support and afford rotation of the front wheel.
Therefore, the pipe like head 11 of the front fork 1 must be round in order to pivotally connect the handle bar stem; the branch pipes 12, 13 of the front fork 1 have to keep an angle "a" in order to straddle and mount on the wheel axle of the front wheel; and the branch pipes 12, 13 of the front fork 1 made of aluminum alloy are made tapered pipes to get the effect of scattering the stress thereon, the tapered pipes are gradually enlarged toward the upward direction to effectively absorb the reaction from the front wheel; and more, the branch pipes 12, 13 are made curved, so that resistive arms are formed between the handle bar and the wheel axle of the front wheel, the resistive arms can provide twisting moment to reduce the force for direction switching. Under these essential conditions, manufacturers can not make the front fork 1 by an integrate forming method due to larger shape variation of the pipe head 11 and the branch pipes 12, 13, therefore, the round pipe head 11 and the branch pipes 12, 13 are always made separately for shaping, so that pipe head 11 is provided on the bottom end thereof with two connecting ends 110 in order to be inserted in the top pipe holes 120 of the branch pipes 12, 13, and then apply adhesive on the junctions, so that the junctions have adhesion joints 14, similarly, the pipe holes 130 on the bottom ends of the branch pipes 12, 13 are also connected by insertion, and the two engaging portions 15 having each a connecting end 150 are fixed with adhesive, in this way, the conventional front aluminum alloy fork 1 is made (as shown in FIG. 1).
Although such adhesive fixing of the connecting ends 110 and 150 to connect the pipe like head 11 and the engaging portions 15 onto the branch pipes 12, 13 is the customary way used in the art, the problem is, there is larger shape variation by bending on the pipe like head 11 and the branch pipes 12, 13, this area is a place where effective scattering of stress is harder, and the conventional adhesive connecting and fixing renders the junctions of the pipe like head 11 and the branch pipes 12, 13 to be subjected to damage of stress concentration; on the countrary, the junctions between the bottom ends of the branch pipes 12, 13 and the engaging portions 15 have smaller shape variation and transferring of stress is smooth although they also are connected by adhesion, so that stress concentration will not induce in these areas; hence as to the front aluminum alloy fork 1 made from the conventional technique, the pipe like head 11 and the branch pipes 12, 13 having larger shape variation by bending are difficult to be integrately formed, this has been the serious impediment for advancing technically to high quality in the art, and this is the motive of providing of the present invention.